Feb. 4, 1930.
1,745,690
E. C. PRITCHETT
MEANS FOB PREVENTING ACOUSTIC SHOCK FROM A TELEPHONE RECEIVER
Filed April 9, 1928
I7 13
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Patented Feb. 4, 1930
1,745,690
UNITED STATES PATENT OFFICE
EDWARD C; PRITCHETT, 0F MOUNT VERNON, NEW YORK
MEANS FOR PREVENTING ACOUSTIC SHOCK FROM A TELEPHONE RECEIVER
Application iîled April 9, 1928. serial No. 268,714.
This invention relates to safeguards for tel
ephone receivers and has for its object to pro
.Referring to the drawings, Fig. 1 is a
wiring diaphragm of this invention;
vide a simple and inexpensive means to elim
inate the hazard of acoustic shock.
5
Inductive disturbances in telephonelines '
Fig. 2 is a longitudinal section through the
copper oxide rectifier;
Fig. 3 is a typical ohmic resistance-voltage 55
are sometimes so serious'as to produce what characteristic for-a copper oxide rectifier.
is called “acoustic shock.” This is a loud noise
An ordinary telephone circuit is shown
comparable to a gun being shot oil' close’to in Fig. 1 comprising the usual transformer l0,
the ear and is thought to be caused’by the transmitter l1, condenser 12 connected in se
10 receiver diaphragm hitting the pole pieces. ries with the receiver 13. Unlike the usual 60
Telephone operators have been rendered un
conscious from such a shock. The most fre
quent cause of such serious inductive disturb
telephone circuit this invention includes a
copper oxide rectifier 14 connected in shunt
with the receiver 13: >In practice the rectifier
ance is a powerful surge such as may be caused 14 would preferably be located in the usual
by a short circuit in a power transmission line coil box adjacent the receiver or wherever con- 65
located close to a telephone line or by light
ning.
venient. The copper oxide rectifier is of the
` general type described by L. O. Grondahl and
There has been considerable investigation P. H. Geiger in the Journal of the American
of means for eliminating this difficulty but Institute of Electrical Engineers for March,
nothing has previously been discovered which 1927, except that those rectifiers having volt- 70
‘ is both effective and sutliciently inexpensive
to be practicable. One suggestion was the
bridging of a carborundum crystal across the
telephone line butthis was not successful be
cause the characteristics of such crystals were
not permanent nor reliable nor were they
ada ted to carry the currents to which they
wou d have to be subjected without break--
ing down. Another disadvantage was that
30 the subjection of such crystal to the telephone
line pressure of about twenty odd volts caused
continuous current flow through them which
age-resistance characteristics given in that ,
article are not as suitable as those having such
characteristics similar to Fig. 3 of the draw
ing herein.
Fig. 2 shows a typical construction for such 75
a rectifier and comprises clamping washers 15
and 16 pressing the oxide filament 18 on a cop
per plate 17 against a sheet of lead 19, the
washers and plates being clamped together
by a bolt 20 and suitably insulated so that the
voltage across the terminals of the rectiñer is
placed across the oxide layer.
soon resulted in evidence of instability of the
Fig. 3 shows a typical resistance-voltage
crystals. A more eíiicient and reliable sug characteristic for the copper oxide rectifier.
" gestion involved the use of a vacuum tube in As the condenser. 12 in the usual telephone cir
- thereceiver circuit connected for use as an
cuit prevents the passage of direct currents
ordinary amplifier in series with the receiver. through the receiver the only normal voltage
The trouble with such an expedient is the across the receiver is the very small pressure
expense and the necessity for inspection and of generally less than .0l volts due to the audio _
renewal due to current consumption in the frequencies passing the’condenser 12. From 90
Fig. 3 it will be seen that the resistance of the
According to this invention a contact rec copper oxide rectiiier at these small voltages
tiíier of the copper oxide-lead type is shunt is enormously high compared with the re
filament.
45
.
'
-
i ed across a telephone receiverrand preferably
sistance of the ordinary telephone receiver,
connected so that the rectifier offers minimum such yreceiver generally being about’î() ohms. 95
resistance to that half cycle of'current whose The exact amount 0f pressure across a tele
magneto motive force adds to the permanent phone receiver at the time of acoustic shock
magnet magneto motive force to attract the is not known but authorities believe it to be
diaphragm toward the receiver pole pieces, if between about 10 or 20 volts or possibly less.
_
' Fig. 3 shows how with increased voltage the 100
f50 only one unit is used.
’
. 2
1,745,690 :- -
resistance of the copper oxide rectifier rapidly t additional advantage of being rugged and re1
decreases to an amount belowthe resistance of atively permanent in its resistance character
the receiver. rl‘he specimen tested and used'Y >istic while at` the same time of suiiicient cur-..
for plotting the curve shown in Fig.ï3 had
a resistance of about 20 ohms with direct cur
rent carrying capacity to be reliable. It has _ ‘
under test withstood currents o_f greater mag 70
rent at 10 volts. At such pressures therefore nitude than wouldbe encountered -in an acous- the maj or portion of current would pass ltic shock from any 60 cycle inductive dis
through the rectifier and >not through the re-' turbance without breaking down. In case of
ceiver. rl‘he rectiñer is preferably „connected disturbances from higher frequencies it has
so that it offers minimum resistance to that half the advantage of being substantially non-in 75
compared with a telephonereceiver
cycle of current whose magneto motive force ductive
so
that
with
such higher frequencies itis bet
adds to the permanent magnet magneto mo-î
tive force to attract the diaphragm toward the ter in its function as a by-pass valve. A cop-'
pole pieces. In other words, it is desirable to per oxide rectifier having roughly about two ' l
connect the rectifier so that its minimum .re square centimeters effective lead plate con 80
10
15
sistance allows most of the current to‘ pass , tact area did not break down until under 1:7
through the rectifier and reduce the likelihood amperes whichv exceeds the severeV acoustic>
of the diaphragm hitting the pole> pieces in
the receiver. To ascertain the- proper wav for 1 While Fig. 2 shows only a single» unit Í'for>
use across the receiver it is desirable for maxi- 85
20 connecting the rectifier, either the direction mum protection to have at least two rectifier
of the windings on the pole pieces are traced
out or a test may be used to ascertain which. units in parallelv and both shunted across the
manner of connection for the rectifier gives. >receiver and oppositely arranged so as/ to' v
least shock in the'receiver on the application offer the minimum resistance to surges in'
either direction.- This last gives more'proV`
-25 of much higher than normal `operatingvolt
tection and -while slightly more expensive
All copper oxide rectifiers will not be found than a single unit is nevertheless believedto '
' alike in their resistance voltage characteris be desirable.
shock- conditions.
age.
»
'
,
p
I
'
,A
A
e
â
tics so that it will be found desirable to test
30
..
The current. carrying capacity .depends on l'
. i
each specimen before being embodied in a tel' >.the effective contact area and' also upon- the ~95
ephone circuit. Some copper oxide rectifiers dissemination ofheat losses in the rectilier.f_
The term “telephone receiver” as used here- y'
have the peak resistance on the left of and
‘i
farther away from -zero voltage than ,does in signifies in'its. broader aspectjan audio" I
`
j 10o
Fig 3. Others will be found to have a much ly one restricted to telephone practice.
less steep resistance gradient. The desirable
the specimen whose characteristic is shown in . electrlc converter for any use and not mereI
40
claim:
`
'
'
-
'
-
rectifier should have say 8 or 1'0 times the
l 1..'l`he combination with a telephone re-l Y A.
phone receivers of much hi her than ordinary
magnet, of a copper oxide rectifier shunted.- '
resistance of the receiver fornormal operat -ceiver, of a'copper oxide rectifier inshunt with` .
ing conditions and a sharp decrease in resist the receiver.
2.l The combination withy >a telephone re-- 105
i
ance for voltages above normal. If it is de
v.celver
having
a
diaphragm
and
permanent'
sired to use the copper oxiderectifierwithtele
resistance“, such as are use in radio work for across the` receiver and arranged to offer.
example, it is desirable that the rectifier be vof minimum resistance to the _component ofvr '
45
correspondingly higher resistance. - To in currents larger` than normal operating cur
crease the resistance of the rectifier the sim-A rents whose dlrection is such that its mag
plest way is to reduce the effective >contact neto motive force adds to the permanent mag- ' _
area of the lead plate. VvAnother way to.- in
crease the rectifier resistance is to add sev
net magneto motive force to attract the dia- .
phragm toward the permanent magnet.
3. The combination with a telephone re
eral rectiliers in series, but for lthis purpose
it is desirable that the resistance-voltage char-. ceiver, of a stable, solid rectifying device in
acteristics of each be of similarA shape. Re shunt with the receiver adapted to safely
ferring to Fig. 3 the dotted line portion of the carry curents under an applied electromotive
graph was not determined by test. From force of 2O volts and -having a decreasing re- '
this
figure it will be seen that the resistance sistance with an increase in impressed voltage 12o,
55
of the rectifier element has decreased rapidly from below one volt, the resistance of. said de- »
vice being proportional to its effective contact
_with increased voltage.
115
v50
The rectifier unit shunted around the re
ceiver functions as an instantaneous pressure
60
responsive relief valve to allow the shock cre
area.
-
‘-
l
Y
4. The combination with a telephone re-`
ceiver, _of means for reducing the hazard of
125
acoustic shock com rising a copper oxide
ating currents to by-pass thereceiver yet of
_fers sufficiently high resistance to the passage rectilier in shunt wit said telephone receiver,
of any current under normal operating condi
tions so as not to affect the successful opera
65
tion of the receiver.- This shunt device has the
sald rectifier having its maximum 'resistance
under an impressed electromotive force of
_ less than one volt.
13ok
1,745,690
5. The combination with a telephone re
ceiver, of means for reducing the hazard of
acoustic shock comprising a copper oxide rec
ti?ier in shunt with said telephone receiver,
said rectifier having a resistance-voltage
characteristic with at least a major portion
of its steeper gradient in resistance on the op
posite side of the zero voltage line from the
10
major portion of its less steep resistance
gradient from its maximum.
6. The combination with a telephone re
ceiver, of means for reducing the hazard of
acoustic shock comprising a rectiñer in shunt
With said telephone receiver, said rectifier
being of the static or contact type Without
moving parts, having a resistance of more
than eight times the receiver resistance under
normal operating conditions, substantially
less than the receiver resistance under ten
20
25
40
50
55
volts, having a steeper resistance gradient un
der less than two volts than a carborundum
rectiñer, and having a resistance which varies
substantially directly with the area of con
tact and which is fixed and reliable.
In Witness whereof I have hereunto set
my'hand this 7th day of April, 1928.
EDWARD C. PRITCHETT.